Known hydrometallurgical methods for separating the precious metals from anode sludge are based on the use of nitric acid, because the solubility of silver as nitrate is high. However, hydrometallurgical processes based on the use of nitrates for treating anode sludge are not compatible with the rest of the electrolytic process, because the electrolytic refining of copper is carried out in a sulfate solution. In addition, the nitrate bearing sludge must be mechanically ground finer, in order to make the leaching successful.
From the publication Hoffman et al. Proceedings Copper 95, International Conference Vol. III, 1995, pp. 41-57, is known a method for processing anode sludge obtained from copper electrolysis. In the said method, the copper and tellurium of the sludge are first leached in an autoclave at high pressure and temperature. After pressure leaching, the sludge is further leached into hydrochloric acid by using chloride gas or hydrogen peroxide as the oxidant. Gold is separated by extraction from the obtained solution. After separating gold, the selenium contained in the solution is reduced by S02 gas. In this process step, also tellurium, gold residues and platinum metals are alloyed. Selenium is distilled from the obtained, precious metals bearing precipitate, and the distilling residue is returned to the process or processed outside the plant. The leaching residue from wet chlorination is processed further for recovering the lead and silver contained therein. After lead separation, the silver chloride from the precipitate is leached into an ammonia solution, reprecipitated as pure chloride and finally reduced into metallic silver.
In the publication Hoffman et al. Hydrometallurgy 94, 1994, pp. 69-107, there is introduced a method for processing anode sludge obtained from copper electrolysis. According to said method, copper and nickel are separated from anode sludge in an autoclave, at a high pressure and temperature. Then selenium is calcinated, and the metals are sulfatized in a calcination furnace. The obtained silver sulfate is converted into nitrate in a ball mill by means of calcium nitrate. Finally silver is separated electrolytically.
One of the disadvantages associated with the above arrangement is that some of the process steps are quite complicated and their control is difficult.